"They are very, very close, if not functionally identical to human embryonic stem cells," said Owen Witte, director of the Broad Center of Regenerative Medicine and Stem Cell Research at UCLA.
Altogether, the researchers were able to create about 10 lines of iPS cells from each batch of 50,000 skin cells.
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"The incredible thing about it is how easy it seems to be," Doerflinger said. "From one little biopsy of a skin sample, you always get enough cells changing to create a stem cell line."
Yamanaka said he could not use human embryonic stem cells in Japan, so he hadn't been able to make a head-to-head comparison between those cells and the new iPS cells. He said he intended to do the tests at UC San Francisco's Gladstone Institute of Cardiovascular Disease.
Even if there were subtle differences in the iPS cells, he said, "I don't think they have to be identical" to embryonic stem cells to be useful in medical applications.
A second team led by Dr. James Thomson at the University of Wisconsin-Madison started with a group of 14 proteins that the researchers suspected would return mature human cells to a pluripotent state. They winnowed down that pool to four proteins: Oct4, Sox2, Nanog and Lin28.
Using their recipe, the Wisconsin scientists were able to reprogram fetal connective tissue cells called fibroblasts. It also worked on fibroblasts from the foreskin of newborns. Their results were published in the journal Science.
Thomson, who isolated the first human embryonic stem cells in 1998, says that the technique is being tested in older cells and that he is optimistic it will work. Other scientists said it might be difficult to adapt the method to cells that were years removed from their embryonic origins.
One benefit of Thomson's approach is that he did not use c-Myc, which has a tendency to cause tumors. About 20% of the mice in Yamanaka's previous study developed tumors attributed to the protein.
The fact that reprogramming could occur without the problematic c-Myc "represents a very important advance," Lanza said.
Both of the discoveries will bring a host of practical benefits. President Bush made most embryonic stem cell research off-limits for federal funding, but the new technique is eligible because it doesn't destroy human embryos. Indeed, Thomson's study was funded in part by the National Institutes of Health.
Money from the California Institute for Regenerative Medicine, which was established to fund embryonic stem cell research, can be used for iPS experiments even though federal support is also available, said Robert Klein, chairman of the agency's oversight board. However, he predicted that studies on embryonic stem cells would win the bulk of the agency's grants.
Scientists, including Yamanaka, stressed the need to continue funding for traditional embryonic stem cell research.
"It would be extremely foolish to place all of your bets on any single approach," said Witte of UCLA.
Some were also concerned that the new approach would prompt a brain drain away from embryonic stem cells, which are still necessary to illuminate the fundamental processes of developmental biology.
Just last week, cloning pioneer Ian Wilmut said he planned to shift his focus from the technology he used to clone Dolly the sheep in 1996 to cellular reprogramming.
"In the long run, reprogramming will be so much more useful and practical," said Wilmut, director of the Scottish Centre for Regenerative Medicine at the University of Edinburgh.
Doerflinger, a longtime opponent of embryonic stem cell research, said he was heartened by Wilmut's decision.
"It would, of course, be ethically unjustifiable to pursue the research using embryos if there were a less morally controversial way to proceed," he said. "Increasingly, it seems that burden has been met."
Though the new technique dodges some significant ethical problems, it also creates new ones, said Insoo Hyun, a bioethicist at Case Western Reserve University in Cleveland. In previous experiments, mouse iPS cells were transformed into sperm and egg cells. If the same can be done in people, he said, "it transforms what we think about human fertility."
Theoretically, the method would even enable people to reproduce after death as long as they banked a tissue sample.
"A person doesn't even have to be alive to create sperm or eggs," Hyun said. "It really is a new technology that brings with it a new set of issues."